Hello from France.
Excuse my bad english ;-)
On MacOS El Capitan, the "startup item" of Abyss Web Server is correctly installed in Library/StartupItemps, but server doesn't launches at reboot.
I taked a look in Activity monitor window, no process "Abyss" launched...
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I tried repairing permissions, without success.
I've disabled SIP, without success.
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For the moment, i just added the application in starting elements of the user account to force server starting, but any help to this issue would be nice.

Just tried on MacOS 10.11.6: the StartupItem works now correctly.
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Many Thanks !
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Always a message when launching the app saying the console is not running on port 9999, but the console works correctly in Safari on port 9999, so this is not a big issue for me.